Data transmitted between a central data and voice communication system such a business communication system (BCS) and a subscriber's data terminal commonly employs a two-wire loop with time compression multiplexing (TCM). A data unit (DU) interfaces the subscriber's terminal to the line and includes status lamps, a keypad, feature keys/lamps, dip switches and other components to provide versatility and flexibility of operation. At the BCS end of the loop, a data line card (DLC) circuit provides a datapath interface between the DU and the BCS.
In order that the DU may provide various service features, and remote access to such features from the BCS, parameters that characterize a DU are generally stored in a volatile RAM. Since the DU is separately powered at the subscriber's location, power loss will occasion erasure of the stored parameters in its RAM.
Although the foregoing problem of memory loss may be avoided by using an EPROM in place of the RAM, this solution is expensive in view of higher costs for a programmable memory and its attendant circuits.
Moreover, the use of EPROMS is expected to result in the inefficient use of a central data and voice communication facility to merely download the DU.